Low power, continuous-wave (CW) Ar+-ion lasers are widely used in applications like bioanalysis, graphics and semiconductor inspection. In many biotech applications, the 488 nm line from the Ar+-ion laser is employed. For this wavelength, particular chemistry has been developed and matched to fluorophores with relatively narrow absorption bands. During use, it is typically necessary to modulate the output from the laser in order, for example, to probe individual samples in an array. Such modulation has been effected primarily by means of acousto-optic modulators. It would thus be desirable to replace these bulky and rather inefficient Ar+-ion lasers with diode-pumped alternatives, particularly for generating said radiation at 488 nm or close thereto.
In general, many applications require coherent light sources of comparatively high power. In particular, this may be the case for high-throughput systems where, for example, a large number of samples should be processed in parallel and/or at a high rate. Furthermore, there is typically a need for light sources having excellent beam quality, in order to provide high reliability for the devices using such light sources. There is also a desire to have small and compact devices, to facilitate mobility and to give a small footprint, and also to reduce power consumption. Additionally, and as mentioned above, there is often a requirement that these light sources can be modulated at any desired rate.
Thus, there is a general need in the prior art for more efficient and stable light sources with an aim of meeting the above requirements and desires, particularly for the visible region of the spectrum.
Visible laser radiation can be obtained in various ways. One approach could be direct frequency conversion of diode lasers (e.g. by second harmonic generation or by sum-frequency mixing). Diode lasers can be manufactured for a wide range of output wavelengths. When using the output from diode lasers for direct frequency conversion, it is often preferred to use periodically-poled non-linear crystals for the frequency conversion in order to obtain high conversion efficiency. However, in order to use diode lasers for frequency conversion in periodically poled non-linear crystals, these diode laser must be single mode. Periodically poled non-linear crystals are, amongst other things, characterized in that they exhibit a very narrow acceptance bandwidth, typically in the order of about 0.2 nm. This leads to high requirements on frequency stability for the lasers generating the fundamental beams for the non-linear interaction. In addition, high beam quality (typically TEM00) is required to achieve high conversion efficiency in the non-linear process. A consequence is therefore that the available output power from the frequency conversion of diode lasers is inherently limited, because high power diode lasers tend to be multi mode both spectrally and laterally, and therefore highly unsuitable for frequency conversion in periodically poled non-linear optical crystals. Also, diode lasers present another drawback in that they have inherently large frequency bandwidths. If diode lasers are to be used for producing the fundamental radiation for non-linear frequency conversion processes, there is a need for some kind of frequency stabilization in order for the conversion process to remain efficient, and even if frequency stabilization is effected the output power is limited due to the single mode requirement described above.
Another approach could be to use second harmonic generation or sum-frequency mixing of diode-pumped solid-state lasers (DPSSLs) in a non-linear medium. The non-linear medium may be, for example, a periodically poled non-linear crystal, such as PP-KTP (periodically poled potassium-titanyl-phosphate, KTiOPO4). DPSSLs are in general excellent in terms of beam quality, frequency stability, line width and power scaling. However, a major drawback of DPSSLs is that they are only available for a limited number of output wavelengths. Also, modulation of DPSSLs is a difficult task, for which typically external devices such as acousto-optic modulators are required.
One example of a DPSSL that may replace the Ar+-ion laser for the 488 nm line is described in the published US patent application US 2004/0125834, which discloses a light source based on sum-frequency mixing of fundamental radiation from a four-level laser with fundamental radiation from a quasi-three-level laser to obtain visible radiation. In one example, radiation at 488 nm is obtained by mixing the output from a Nd:YLF laser operating at 1047 nm with the output from a Nd:YVO4 laser operating at 914 nm. However, this solution only solves the problem of obtaining some specific wavelength, e.g. said line at 488 nm. Moreover, modulation of the output from such a laser can be cumbersome and may require costly auxiliary devices and/or lead to an overall reduction of output power.
A still further approach that has been proposed is to combine a diode laser and a DPSSL, and to generate visible coherent radiation by sum-frequency mixing of radiation from these two light sources.
U.S. Pat. No. 4,879,723 discloses a method of generating coherent optical radiation by sum-frequency mixing, wherein radiation of a first frequency is generated within an optical cavity by optically pumping a lasant material, radiation of a second frequency is generated from a diode laser, radiation of said second frequency is introduced into said optical cavity, and sum-frequency mixing is performed with a non-linear optical material within the optical cavity to produce optical radiation a third frequency which is the sum of said first and second frequencies.
U.S. Pat. No. 4,791,631 discloses a process for producing coherent radiation at essentially 459 nm by mixing, in a non-linear crystal of KTP, two fundamental laser beams, one at 808 nm and the other at 1064 nm. The idea is to be able to employ non-critical phase-matching in the KTP crystal.
U.S. Pat. No. 5,142,542 discloses sum-frequency mixing of two fundamental wavelengths, wherein the non-linear process takes place within a cavity that is resonant for both the fundamental wavelengths.
U.S. Pat. No. 5,412,674 also discloses sum-frequency mixing of two fundamental wavelength, particularly by means of a non-critical phase-matched KTP crystal.